This invention relates to a digital data transmission device for a composite transmission signal comprising a main data and a subdata signal by using a multilevel encoding system and to a digital data reception device for use as a counterpart of the digital data transmission device. Such a digital data transmission device is particularly useful in a data communication system which utilizes a telephone line.
Frequency division is often used in the digital data transmission device in transmitting the subdata signal together with the main data signal. The frequency division digital data transmission device comprises a frequency division multiplexer. The frequency division multiplexer multiplexes the main data signal and the subdata signal into the composite transmission signal. The composite transmission signal is transmitted in a transmission frequency band. In this event, it is necessary to allocate a main frequency band and a subfrequency band for the main data and the subdata signals in the transmission frequency band. A guard band must be used in separating the subfrequency band from the main frequency band. This means that the digital data transmission device of the type described above has a reduced transmission capacity for the main data signal because the main frequency band must be narrowed due to use of the subfrequency band and the guard band. Such a digital data transmission device is not suitable for a data communication system which utilizes a telephone line and is put in operation with its maximum possible transmission capacity.